Dev-Mode Debug Coordinate Overlay, Spreadsheet-Style Coordinates

A toggleable debug overlay (the "🐞 Debug" button) draws a faint checkerboard tint plus a bold, readable coordinate label over every grid cell — legible on a vertical side-monitor at a glance, after direct user feedback that an early pass was too small/yellow to read there. Coordinate format converted from raw numeric pairs to spreadsheet-style column-letter + row-number (A1, B6, ...) per the workspace-wide ../AGENTS.md convention for grid/tile-based UIs.